Wire bonding processes, especially on Lead-Over-Chip (LOC) devices, are sensitive to the shape of the lead tip, leading to a significant frequency of "slung wires" which in turn can lead to yield loss and impaired reliability of the assembled IC's. In the case of LOC devices, the stitch bond to the lead tip is substantially above the plane of the chip/ball bond, resulting in a loop shape as the bond capillary approaches the second or stitch bond to the lead finger. When this loop touches the end of the lead, there is a strong tendency for the wire to be deflected laterally just prior to capillary touchdown, resulting in a slung wire. "Slung wire" is a term used to describe an IC assembly fault resulting from a lateral deflection of the interconnection wire from its intended path. Reduced clearance between adjacent wires are potential performance and reliability problems. If the lead tip is rounded, as is generally the case with chemically etched lead frames, the slung wire probability is greatly increased. Conventional methods of designing photomasks to produce etched lead frames cannot eliminate this rounding tendency, especially as lead pitch or center to center distances become smaller to accommodate higher density devices.
Production of lead frames by stamping, as opposed to chemical etching, results in lead tips that are cut square, reducing the tendency to have deflected or slung wires at assembly. This, however is cost justified only on stable high volume designs with adequate product life. Many new and high density products are not well suited to immediate conversion to stamping, so an etch solution is desirable.
Other conventional solutions involve very special and sometimes complex wire loop paths, which can imply more gold wire consumption and reduced productivity.